Interleaving is a technique used in data communication systems for protecting codewords or other data words against burst errors that occur during data transmission. Several consecutive bits of a data stream are corrupted during transmission when a burst error occurs, e.g., due to impulse noise. Error correction schemes that expect errors to be more uniformly distributed can be overwhelmed when burst errors occur. Data is often transmitted with error control bits that enable the receiver to correct a certain number of errors that occur during transmission. However, if too many errors occur within a codeword, the codeword cannot be correctly decoded despite use of an error correction scheme. To mitigate burst errors, the codeword bits are typically interleaved before being transmitted. This way, bits from the same codeword are not transmitted in consecutive order. Instead, the codewords are broken up and the bits from different codewords are consecutively transmitted. A burst error is much less likely to corrupt a codeword because the codeword bits are more uniformly distributed during data transmission. Receivers are more capable of correctly decoding transmitted codewords when interleaving is used.
Many types of data communication systems that employ interleaving are provided a fixed amount of memory to be allocated between interleaving and de-interleaving operations. For example, DSL (digital subscriber loop) modems such as VDSL (very high speed DSL) and VDSL2 modems are typically provided a total memory size (in terms of delay octets) to be allocated between a downstream interleaver buffer and an upstream de-interleaver buffer. Codewords are interleaved in the downstream direction (i.e., from a service provider or operator to a subscriber) and de-interleaved in the upstream direction (i.e., from the subscriber to the service provider or operator). The protection capability of the interleaver depends on the size of the interleaver buffer. The amount of interleaver buffer memory (interleaver depth) is typically proportional to the data rate, maximum delay and the required minimum impulse noise protection for the downstream and upstream flows.
The fixed memory is ideally allocated so that a desired noise protection capability is achieved while maintaining optimal upstream and downstream data rates. However, the optimal upstream and downstream data rates depend on actual channel conditions and interleaver memory allocation is typically performed before channel conditions are known to the modem. For example, DSL modems typical allocate memory between interleaver and de-interleaver buffers based on data rate, maximum delay and impulse noise protection configuration parameters provided to the modem as part of the MIB (management information base). These parameters represent worse-case, best-case or expected channel conditions. Therefore, the memory may not be optimally allocated between the interleaver and de-interleaver under some conditions.